How to add bumpiness to a car/roller coaster motion?
You could do this by adding the noise modifier to you animation.

1) Open the property’s tab in the graph editor and select the animation you want to add the bumpiness to
2) Go the modifiers tab and select noise

You can read more about the noise modifier and other fcurve modifiers here

Note: Don’t add the modifier to the f-curve that controls the direction your object is going. For this particular problem add keyframes to your Y and Z locations of the car (assuming that car is moving along the X direction of the path) and then add noise to f-curves for Y and Z locations.